The present invention relates generally to drive and control of electromechanical motors or other actuators having capacitive motor/actuator phases, and in general to methods and devices for reducing the power consumption of such motors or actuators.
There are numerous applications with the need for miniaturised motors that are able to make controlled fine positioning. Among these, portable devices, such as cameras, telephones and portable computers, have additional demands for low power consumption, low weight and price.
Electromechanical motors, e.g. piezoelectric motors using repetition of mechanical steps, are potential candidates in these applications. One of the present drawbacks is the low efficiency due to energy losses in the drive electronics. Previous solutions have included mechanical resonance in the piezoelectric components, which gives a certain energy saving possibility, at least in theory. For fine positioning and in particular linear motors, resonant motors are not ideal and e.g. inertial or quasi-static drive mechanisms are preferred. It is possible to use electric resonance to reduce the power losses, but it reduces the possibility to optimise waveform shapes and to position at fractions of steps. The motors that are able to make controlled quasi-static mechanical stepping are so far driven with waveform generators with no energy saving capacity.
Electromechanical motors, such as the piezoelectric motors, have a number of drive elements comprising portions that change shape in accordance with the applied electric voltage. From an electrical point of view motor phases comprising drive element portions are capacitive, and the common solution to drive these capacitors is to use an amplifier circuitry. Basically an analogue control signal is used as an input signal to an amplifier, which provides the appropriate charging/discharging voltage to the motor phase. When charging up a motor phase, all current originates from the energy source of the amplifier. During charging of the motor phase from zero to the voltage of the energy source, it is easily shown that the energy losses in the amplifier and connectors are at least xc2xdCU2, where C is the capacitance of the motor phase and U is the energy source voltage. When completely discharging the motor phase to ground, another loss of xc2xdCU2 is experienced. This means that in every charging-discharging cycle, a total energy amount of at least CU2 is lost. Since the operating frequencies for electromechanical motors is typically in the kHz range, the total energy consumption becomes large. Most of the losses are converted into heat in the electronics parts of the devices, hence the drive electronics normally requires relatively large volumes. This is of course disadvantageous for miniaturised devices. Further, in battery driven devices, high losses will result in reduced operation time.
One way to decrease the loss amount is to reduce the operating voltage and/or capacitance of the motor phases. However, this will obviously influence the performance of the motor phase in a disadvantageous manner.
Some solutions of how to reduce energy losses in the drive circuits for motors or other actuators with capacitive loads have been presented, see references [1] and [2]. Common to these solutions is that an inductive component is used to store the energy during the energy transfer. The drawbacks with energy saving based on inductive components are the non-negligible volume of low-loss inductors and the need for advanced control algorithms during charging and discharging. In applications where the total volume of motor and drive electronics needs to be minimised, a solution with no or extremely small external components is desired. Further, complex control algorithms will put particular demands on the control electronics increasing both price and physical size.
In reference [3] the usefulness of an inductor in driving piezoelectric motors has been presented. During a discharge operation, a switch is closed for a period of time, in order to build up a current in the inductor. The current is then directed to a power supply, by opening the switch, making use of the hereby induced voltage in the inductor. Unfortunately, switch control timing is not entirely simple and efficient inductors are rather voluminous. In practise, this inductor-based design has not yet been adopted for use with miniaturised piezoelectric motors.
A general object of the present invention is to provide methods and devices for reducing energy losses in the drive electronics of electromechanical motors or other actuators having capacitive motor phases. Another object of the present invention is to reduce the volume of the drive electronics. A further object of the present invention is to provide less complex control means for the drive electronics of electromechanical motors. Yet another object is to provide operating voltages for the motor phases, which exceed the voltage of the power supply.
The above objects are achieved by methods and devices according to the enclosed patent claims. In general words, charging and discharging of motor phases in an electromechanical motor is performed with small voltage difference between the voltage source and the capacitive load. Energy from discharging operations is stored to be used in subsequent charging operations. The voltage sources are preferably provided by means of capacitive voltage step-up or step-down circuits, whereas switches control the charging and discharging events.
One advantage with the present invention is that the energy losses are reduced to a fraction of the losses for transistor-based prior art devices. A further advantage is that the volume of the drive electronics now can be made very small. Yet another advantage with step-up circuit embodiments is that the motors can be driven by low voltage power supplies.